Vortex flowmeters are known in the art. They can be used to measure the mass flow rate (typically used for gaseous delivery conduits) or volumetric flow rate (typically used for liquid delivery conduits) of a fluid flowing through a conduit. The mass flow rate is equal to the density of the fluid flowing in the conduit times the velocity of the fluid times the cross-sectional area of the conduit. The density can be treated as a constant with a value of the density provided in advance for the calculation, in the cases where density is a known constant, such as for liquid delivery systems. Alternatively, the density can be calculated in a known fashion from an equation of state such as the ideal gas law, if the pressure and temperature of the gas can be sensed. Accordingly, some vortex flowmeters also include sensors for sensing the pressure and temperature of the fluid in the conduit. The volumetric flow rate is equal to the velocity of the fluid times the cross-sectional area of the conduit.
As can be seen, the calculation of mass flow rate and volumetric flow rate each require a determination of the fluid velocity. Fluid velocity is typically measured in vortex flowmeters by inserting a bluff body, or shedder bar, into the flow of turbulent fluid and counting the frequency of the vortices produced thereby, since the frequency of the vortices is proportional to the fluid velocity for well-designed flowmeters. As shown in FIG. 1, a conduit 20 with fluid flowing therethrough in a direction shown by an arrow 22 will form vortices after passing a bluff body 24 placed in the conduit 20. The vortices will be alternately created in wake 26 or wake 28 formed on opposite sides of the bluff body 24. Each of the wakes 26 and 28 are composed of a series of vortices 30. The vortices 30 of wake 26 rotate counterclockwise, while the vortices 30 of wake 28 rotate clockwise, as seen in FIG. 1. The vortices 30 are generated one at a time, alternating between the opposite sides of the bluff body 24. The vortices 30 interact with their surrounding space by overpowering every other nearby swirl on the verge of development. It is known in the art that the distance (or wavelength) between successive vortices is constant, within a given distance downstream of the bluff body 24. Since the distance between successive vortices is constant and the inside diameter of the flow conduit is constant, the three-dimensional volume of fluid between the vortices is also constant. By sensing the number of vortices passing by the sensor in a given time, the vortex flowmeter can compute the total volume of fluid which is passed through the conduit in that same given amount of time.
It is important for the fluid flow through the conduit to be turbulent rather than laminar. Turbulent flow is determined using the well known dimensionless number called the Reynolds Number: ##EQU1##
where
Re=Reynolds Number PA1 .rho.=mass density of the fluid being measured PA1 V=velocity of the fluid being measured PA1 D=internal diameter of the fluid conduit PA1 .mu.=viscosity of the fluid being measured PA1 St=Strouhal Number PA1 f=frequency PA1 d=equals width of the bluff body PA1 V=equals fluid velocity
The Strouhal Number is the other dimensionless number that quantifies the vortex phenomenon. The Strouhal Number is defined as: ##EQU2##
where
Well-designed vortex flowmeters exhibit a constant Strouhal Number across a large range of Reynolds Numbers, indicating a consistent linear output over a wide range of flows and fluid types. Below this linear range, intelligent electronics automatically correct for the variation in the Strouhal Number with a Reynolds Number. Known smart electronics correct for this non-linearity by calculating the Reynolds Number based on either constant values of the fluid's density and viscosity stored in the instrument's memory or measured values of pressure and temperature in an equation of state (such as the ideal gas law) for density in an equation to predict viscosity.
Vortex flowmeters can be used in either in-line or insertion applications. In in-line applications, the flowmeter body includes a section of fluid conduit which may have flange connections at opposite ends for connection to opposed ends of an existing fluid conduit. Insertion type vortex flowmeters may include a shroud which houses a bluff body and a sensor that can be inserted into an existing fluid conduit or pipeline via an opening on the radial wall of the fluid conduit.
While there are many vortex flowmeter products available on the market today and disclosed in the patent literature, it is believed that there are none that optimally fit the following requirements for a vortex flowmeter: high sensitivity (expressed as the ratio between the maximum and the minimum measurable flow rates, also known as the turndown ratio), rugged, able to withstand pressure fluctuations, substantially free from sensitivity to vibration, inexpensive, reliable, and compact. In particular, it is desirable to design a vortex flowmeter with improved signal-to-noise ratios, a decrease in the sensitivity to vibration, and with a design that can be easily and cost-effectively manufactured. For example, there are other vortex flowmeters available today that may satisfy certain of these parameters but may be relatively difficult to manufacture. For example, some vortex flowmeters require sensors that are difficult to mount, wires that are difficult to connect to the sensors, and designs that require the potting of the sensor into a sensor tube.
It is against this background and the desire to solve the problems of the prior art that the present invention has been developed.